Birth control help

So I’m 17 and my bf and I have been talking about sex. I want to be safe and not end up pregnant so I have been asking my mom for birth control. The other day she told me that she will make a doctors appointment for me however she would be extremely disappointed. I am a virgin and this is my first serious relationship and I grew up in a very Christian household. How can I have her set up the doctors appointment without her becoming disappointed/ get cheap (bc im broke literally have like $2) birth control without her knowing?